Many telephone systems, particularly cordless telephone systems, are designed to operate in urban environments which have a high level of ambient noise. Conditions of this type make the optimization of sidetone level extremely important. In telephony, sidetone is defined as an attenuated level of one's own voice heard in the telephone handset ear piece. High sidetone levels are not only annoying to the user but can, under certain conditions, severely degrade intelligibility. The level of sidetone is dependent to a great extent upon the telephone line conditions unique to each installation. A mismatch between the telephone line and the telephone instrument can cause a high level of sidetone which is disturbing to the user and adversely affects intelligibility. Some sidetone is desirable to make the user aware that the telephone is active. However, a loud operating environment often necessitates the use of high receive volume levels in a cordless handset, with the result that the sidetone level increases to the extent that the background noise in the ear piece reduces intelligibility.
Some telephone exchanges, such as public switched telephone networks, use digital signal processing (DSP) techniques to remove the transmitted speech from the receive path however, this is a costly approach for a low cost consumer product. Hence, there is a need for a cost effective circuit and technique that reduces sidetone in a handset.